The invention relates to an apparatus with two plates, which are spaced apart from each other and are movable in relation to each other and which define an intervening space for receiving a substance to be tested, whose rheological, particularly viscometric, properties can be determined simultaneously with another measurement procedure that can be carried out by the apparatus.
An apparatus of this general type is disclosed, for example, in published European patent application no. EP 750,186 in which the rheological properties of a substance are determined between two plates. For this purpose, the bottom plate of two substantially parallel plates is driven and set into rotation. As a result, the substance is subjected to stress, particularly shear stress, and a sensor forwards a signal to a control unit. At the same time, heat is applied to the substance and an additional sensor is used to determine thermal characteristics.
Carrying out the two measurement procedures simultaneously has two essential advantages compared to carrying out two separate measurement procedures. On the one hand, the two procedures can be carried out simultaneously on the same sample so as to exclude possible error influences due to non-homogeneities in the substance. Furthermore, any possible changes that the substance may undergo in the interim when the two measurement procedures are carried out can also be avoided. On the other hand, the two measurement procedures can be subjected to a comparative evaluation, so that the measured data can be checked. This makes it possible to detect any interfering environmental influences, which are then excluded from consideration in the further analysis.
The two measurement methods are not simply modifications of the same measurement principle but provide insights into the properties of the substance by independent routes. The corresponding information, however, makes it possible to draw conclusions about the results of the respectively other measurement method.
However, in carrying out such rheological tests, the high demands placed on the surface quality of the plates are problematic. As a result, it is not possible, for example, to arrange sensors in the intervening space between the plates or on the surface of the plates, because even the slightest changes in the surface quality can lead to errors in the rheological test.
It is also know to use optical spectroscopic measurement methods for testing substances. The substance is measured either in transmission or in directional or diffuse reflection. While specific conditions have to be met for measurement methods using the reflection principle, transmission measurements, particularly in infrared spectroscopy, require a time-consuming preparation of the substance. For example, complicated microtome sections or molten films have to be prepared for this purpose.
In contrast, a measurement method based on the principle of attenuated total reflection (ATR) offers an almost universal means for rapidly and simply analyzing a wide variety of substances. The ATR method is based on substance-specific infrared absorption at the interface between a medium with a high refractive index and the substance, which has a relatively low refractive index. For this purpose it is sufficient to provide adequate contact between the substance and the ATR element. The principle of ATR is based on injecting an infrared beam into a crystal material that is transparent to infrared light and whose refractive index is greater than the refractive index of the substance. On its path through the ATR element, the infrared beam is totally reflected one or more times. At the points where the beam contacts the interface between the ATR element and the substance, substance-specific portions of the infrared beam are absorbed. This produces a spectrum that is very similar to transmission. The ratio between the total reflection and the attenuated values is determined as a function of the transmission across the wavelength of the infrared radiation. One advantage compared to transmission measurement methods is that the layer thickness of the substance is irrelevant in the ATR method. Because of its enormous advantages regarding substance preparation, the ATR technique, today, has largely replaced the transmission methods.
U.S. Pat. No. 5,905,196 A discloses a viscometer based on the principle of the emission method. The rheological stress of the sample generates friction heat, and the corresponding thermal radiation is measured without contact and without influencing the measurement process itself. For this purpose a window is provided. A necessary condition for carrying out this method is that the infrared thermal radiation is not influenced, i.e., particularly not attenuated, as it passes through the window. As a result, the window has to be highly transparent.
The German publication DE 689 19 256 T2 discloses a transmission method in which the light beam passes through the substance parallel to the lower plate or the upper cone. The transmission path thus corresponds approximately to the diameter of the cone-plate arrangement. A drawback is that the described arrangement results in a comparatively long transmission path, which substantially limits the selection of the wavelength range of the light.